1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive to estimate repeatable runout (RRO) based upon an optimal mean square estimation (MSE) learning method.
2. Description of the Prior Art and Related Information
In order to keep up with consumer demand for disk drives with greater memory and smaller form factor sizes, disk drives are being manufactured with ever-increasing storage capacity in terms of the number of bits that can be recorded on a given track of a disk as well as the number of tracks that can be written on a disk (often referred to as track density).
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes a disk having a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the moveable transducer head.
The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges that are angularly spaced from one another and are interspersed between data sectors around each track of each disk.
Each servo wedge may includes a phase lock loop (PLL) field, a servo synch mark (SSM) field, a track identification (TKID), a wedge ID field having a binary encoded wedge ID number to identify the wedge, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a course “seek” mode using the TKID field as a control input.
Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a fine “track follow” mode. The transducer head generally reads the servo bursts to produce a position error signal (PES) that is 0 when the transducer head is at a particular radial position.
The general goal of the servo control system is to control the transducer head position relative to a desired position—i.e. to get it there and to keep it there. There are numerous outside influences, which make it difficult for the servo control system to achieve the desired transducer head position, but a particularly troublesome influence is known as “runout.”
Runout generally refers to deviation from perfect circular motion and, more particularly, refers to variation in the distance between an external point of reference and a passing surface of a rotating object. “Repeatable runout” involves periodic deviations that occur with predictable regularity (hereinafter “RRO”). “Nonrepeatable runout” involves random perturbations due to, for example, resonance modes, disk flutter, windage, disk vibrations, and so on (hereinafter NRRO).
In the context of a disk drive, RRO is “repeatable” because it occurs in synchronization with the spinning disk. RRO comes from one or more of the following sources: a) spindle motor runout; b) disk slippage; c) disk warping; and d) disturbances converted to RRO during the servo-writing process due to, for example, vibrations, resonances, media defects, or disk distortion due to clamping of the HDA. RRO that is written-in to the disk during the servo-writing process may be referred to as written-in RRO (WRRO). WRRO and other repeatable disturbances may be collectively referred to as RRO. RRO appears as a component of the PES.
Determining and canceling RRO has become more important as higher track densities in disk drives are achieved. Particularly, determining the RRO of a disk for use in track following is important because excessive RRO may cause track mis-registration (TMR) problems. In fact, RRO may ultimately lead to an upper limit on achievable track densities, as RRO cuts into the available track misalignment budget. Methods of determining RRO and then canceling it by one of a variety of techniques are already well known in the art.
For example, a common method of determining RRO is a time averaging method. In this method, during disk drive manufacture, PES values are averaged for servo wedges of a track over many revolutions during a dedicated RRO learning process. Determined RRO values may then be written in the servo wedges. In subsequent disk drive operation, during servo track following, the RRO may be compensated for by subtracting the stored RRO value such that a smaller PES value is achieved and fewer TMR problems occur. A conventional averaging algorithm that is commonly utilized is:
            1      n        ⁢                  ⁢                  ∑                  i          =          1                n            ⁢                          ⁢              PES        ⁢                                  ⁢                  (          n          )                      ;wherein n is the number of revolutions of the disk of the disk drive and PES (n) is the collected PES on all wedges of a track at an n-th revolution.
The dedicated RRO learning process is presently performed for each disk drive during disk drive manufacture and it is a time consuming process that introduces a great deal of cost to the manufacture of the disk drive. Therefore, techniques to improve the speed and accuracy of the RRO learning process are constantly sought after.